The present invention relates in general to on-board aircraft sensing and/or warning systems. In particular, the present invention relates to a system for warning the flight crew on board an aircraft of pre-flight icing on an outer skin of an aircraft structure, critical lifting surfaces, or control elements. The warning extends to the susceptibility to icing or snow accumulation during loading, taxiing, or takeoff.
Aircraft icing continues to be a hazard that affects lives and property wherever air transportation is used. A brief discussion of aircraft icing, in the context of in-flight icing, appears at page 57 of Van Nostrand's Scientific Encyclopedia, Sixth Edition, Douglas M. Considine, P.E., editor, Copyright.COPYRGT. 1983. The airspace in which aircraft icing is most commonly encountered lies between the earth's surface and 20,000 feet (6000 meters). Several factors are involved in icing; i.e., the amount of supercooled water in the space swept out by the moving plane, the speed of the plane, drop and droplet size, and the temperature of the airspace.
Where drops and droplets freeze instantly, the water material does not have time to form a crystalline structure and the ice formation tends to be opaque, often granular with occluded air, usually less dense than clear ice, and frequently an irregular surface. This is called rime ice. When the liquid droplets and drops have time to crystallize, the ice formation tends to be more or less clear, usually near the density of ordinary ice and tends to assume the shape of the external aircraft surfaces over which it tends to spread as it freezes. This is called clear ice. Temperatures just less than freezing are conducive to clear icing and temperatures well below 0.degree. C. (32.degree. F.) are associated with rime icing. Aircraft icing is usually a mix of the two types with the proportion of the two related to the temperature.
The article concludes by stating that aircraft icing at one time in aviation history was a very serious problem, usually in winter, to all aircraft, and that it continues to be a serious problem for many smaller aircraft. The reason given for this distinction is that the current generation of transport and military aircraft are capable of nullifying the presence of icing conditions in numerous ways. Heated leading edges and surfaces can be built into the aircraft structure. Also, jet-engine aircraft tend to operate not only at speeds which produce a thermodynamic warming of the air immediately above the leading surfaces and edges, but also at altitudes well above 20,000 feet (6000 meters) where clouds are predominantly composed of ice crystals (an environment less conducive to icing). However, smaller propeller-driven aircraft, even those equipped with anti-icing devices, tend to be subjected to icing hazards in airspace where supercooled clouds and rain are present (an environment more conducive to icing).
It is clear that aircraft icing is still a problem. However, it is the present inventor's experience that the above article neglects a fundamental aspect of the real problem created by aircraft icing, namely that it is most dangerous during initial takeoff. Recent accidents at major airports around the world are clear proof that undetected, pre-flight icing conditions on the critical lifting surfaces of an aircraft can have deadly consequences.
At present, pre-flight icing detection is still a relatively primitive procedure. Weather reports advising of conditions ripe for icing usually lead to an actual visual or tactile inspection of the critical lifting surfaces. However, the size and extreme wingspan of today's modern aircraft often makes it difficult if not impossible to adequately visually inspect all the critical lifting surfaces. The pilot wants to know if the aircraft critical lifting surfaces are icing up, but once in the cockpit his ability to visually inspect these surfaces is limited. The pilot then has to rely upon the ground crew to an even greater degree. This problem becomes even more exacerbated at night or during low visibility conditions due to fog, snow, and the like. Even very little icing can be a serious problem.
If pre-flight icing is detected, and the aircraft is still at the main airport facilities, airport personnel implement their standard pre-flight deicing procedures. Various liquids using alcohol or similar substances are sprayed over the aircraft critical lifting surfaces and are effective in removing accumulated ice, and preventing its return for a limited period of time. However, if the aircraft taxis out towards the runway and encounters a delay which stacks up several aircraft during inclement weather, the deicing protection begins to runout. If the pre-flight delay is too long, the pilot will have no choice but to abort the flight, head back to the deicing area, and have the aircraft sprayed down again. While the value of human lives and property certainly warrant these efforts, a significant portion of airport operation delays and expense can be traced to these pre-flight deicing procedures. An unnecessary deicing procedure costs the same as a necessary one, and one can appreciate the magnitude of the problem if the cost of this one delay is multiplied by the dozens of airports around the world experiencing these conditions.
It is thus apparent that a system for warning the flight crew on board an aircraft (or the ground crew attending the aircraft) of pre-flight aircraft icing, i.e., of whether an outer skin of an aircraft structure is icing or is susceptible to icing, would be of great practical benefit not only to the airline industry, but also to the passengers and crew traveling aboard such aircraft.